Suppose I’m given a pair of binoculars. I look through them at something, and it’s fuzzy. So I rotate the little knob on top of the binoculars. I turn it one way, the image gets fuzzier, so I turn it the other way. It gets sharper. By making minute analog adjustments, correcting for small errors, I can get the image as sharply focused as I’d like. And after I’m done, somebody could probably use the position of the focus wheel to determine how far away the object I was looking at is.
Then I go to an optometrist and sit in his chair. There’s an image in front of me; it’s fuzzy. He sticks some lenses in front of my eyes. Does he give me a little focus wheel, so I can turn it and bring the image into sharp focus? Noooooooooooo. Instead, it’s a whole series of “Which looks clearer, this? Or (flip) this? This? Or (flip) this? This? Or (flip) this?” Over and over and over again, without any opportunity to see both images simultaneously, without any opportunity to make minute adjustments. Oftentimes, the two images look very similar, and since they can’t be seen at the same time, it’s guesswork. It seems a horribly inefficient and inexact way of figuring out what sort of optics are needed to bring an image into focus.
I know optometrists aren’t stupid. There must be some reason for this apparently-stupid method, but what is it?
A big part of the reason is that there isn’t a single “dial” that optometrists are turning.
My optometrist actually starts the session by looking through my eye and adjusting settings until the retina looks clearer to him, which I guess gets him in the ballpark. Then it’s a matter of adjusting different settings that does require the doctor to do it. I have bad astigmatism in my left eye (8 or so diopters) and part of the procedure is to figure out not only the number of diopters, but the orientation of the astigmatism. When you’re adjusting diopters, you need to snap in different lenses, so a smooth dial motion wouldn’t be possible with current technology.
Often, it’s impossible for me to tell whether A or B is better - they’re both mediocre, but in different ways. (For example, which is better: a blurry single image or a sharper double image? Beats me.) My doctor says that as long as I’m communicating what I see, we’re making progress, and that if I can’t tell the difference between two settings, I should just say so.
He’s not just making adjustments in the focus. He’s also checking for astigmatism and such. It’s a little more complex than binoculars. If you can’t tell, you can say so or ask to do it again.
All you’re adjusting with the binocular’s focus wheel is, if I understand it correctly, the focal length, that is, how far away from the front of the lens farthest from your retina the image comes into focus.
This is sufficient if your eyes are reasonably equal in vision and normal.
In the chair with the “this or this,” you’re getting a much more comprehensive analysis of your vision which can actually diagnose things like astigmatism, myopia, presbyopia, and other fun things. It’s not just about making the image sharp.
A good optometrist will not say simply “which is better, this or this” although I’ve had many that do this.
My last one knew exactly what I was looking at and asked for more discrete differences such as “sharper but smaller?”,“smaller but darker?”,“blurry or ghost imaged?”
I’m not an optometrist, but I can think of several factors:
It would be possible to make a variable-focus lens and let you adjust the dial. But it would be very difficult to keep such an instrument in proper alignment over the course of many years. If you just have a set of lenses, and use a combination of them, a mechanical error cannot cause an incorrect result.
The prescription usually consists of 3 parameters for each eye, so it’s a bit more complicated than turning one dial to find the best setting. The optometrist has far more experience on how best to arrive at the best setting.
The optometrist wants an objective, unbiased information on what looks best, without your knowing what the settings are. If he let you play with a dial, the “best setting” you find may not actually be the best. There may be a sort of placebo effect - e.g. if you find that turning one dial one way makes the image really fuzzy, you may over-compensate by turning it too far the other way and convince yourself that that’s the best setting.
How do you expect to see both images simultaneously? You’re comparing two different settings for the same eye.
I can see some images well if I stare for about a second but I cannot see them clearly when the optometrist is switching back and forth quickly. I attempted to halt a test once and get the person to explain but she refused to acknowledge my questions.
Switch optometrists, jane. They should be willing to let you see the comparison again or slowly enough for you to judge.
Isn’t it also that lenses will come in discrete measurements and not down to the minutest difference? They’ve gotten it down to where two look essentially the same to me, but neither is perfect, but that’s as close as they get.
Consider what’s being adjusted - when you focus the binoculars, you are adjusting the distance between two or more lens elements which provide the magnification, to obtain the correct focus. The optometrist is trying out different actual shapes for a single lens. Mechanically difficult to make that a reliable continuous adjustment. And, as pointed out above, if you look at your prescription, you will note that it (usually) consists of three numbers. Even if such a mechanism were possible, it would be complicated by having to make all those adjustments to the lens shape independently.
He also repeats them if you are unsure. I asked my optometrist. Sometimes he tells you he’s repeating, sometimes he just repeats them later without saying anything.
One other thing to consider: Binoculars are typically used outdoors in daytime. In those conditions, your pupils are contracted, which makes your eyes very tolerant of focus error and other aberrations. The contracted pupil effectively reduces the size of the blur. (It’s the same reason why squinting your eyes often helps you see better.) So it’s usually very easy to focus a pair of binoculars.
But that would be bad for checking your eyes; you want your eyes to be as sensitive to focus error as possible. Which is why optometrists’ offices are so dimly lit. Under those conditions, if you were given a pair of binoculars or adjustable lenses, I think you’ll find it very difficult to find the optimal focus. It’s the same when using astronomical telescopes - I’d typically move the knob back and forth dozens of times before I’m convinced I’ve found the optimal focus.
Short, perhaps oversimplified answer: you turning the knob until it looks “sharp” can very, very easily lead to over-minusing, which leads to you overfocusing. You’ll like how that looks in the exam room (all sharp and crisp!) and then go home and try to read a book or use the computer or do something close-up that requires even more increase in focus, and pretty soon may experience the drawbacks of constant over-focusing – headaches, double vision, intermittent blur. Quite a lot of time is devoted to teaching the student optometrist (and I am one, so I know) how not to over-minus patients, particularly young ones who can accommodate (i.e. increase the refractive power of the eye by changing the shape of the crystalline lens) easily.
Also, binoculars don’t need to adjust for astigmatism, which complicates the equation. Nor do they need to change the Rx when a different prescription for near-work is required.
“Overminusing” means using a stronger minus lens (or a weaker plus lens) than is required for your ideal prescription. This, in effect, turns a myope into a hyperope (or a hyperope into a slightly-better corrected one.)
Ever borrow your friends’ glasses in school and think “Ooh, everything looks really sharp!” They probably had a stronger negative prescription. In the exam room, it can make letters look “crisper” up to a point, but also smaller, since minus lenses minify images. Someone with good accommodative ability can accommodate through the minus and probably won’t be bothered in the distance.
When the ciliary muscle contracts, changing the shape of the crystalline lens, you increase the refractive power of your eye, thereby “counteracting” the minus lenses. But the ability to accommodate is also required in order to see close up. You therefore have that much less of an ability to accommodate.
Some people may not be bothered by this, and some will. Symptoms, as I’ve mentioned, include blurry vision (potentially from accommodative spasm, resulting from the constant over-accommodation, or, when dealing with patients on the cusp of presbyopia, blurry vision at near due to “running out” of accommodative amplitude,) headaches, and double vision (which can be brought about by the neurological linkage between accommodative and convergence systems – when you over-accommodate, you also have a tendency to over-converge.)
In my experience at least, younger patients are less likely to be bothered by over-minusing equally in both eyes (although some are) and are more likely to be bothered if one eye is over-minused or if the two prescriptions are not in some way equally balanced, because your eyes tend to accommodate equally, and in that situation a blurry image will always be present in one eye no matter how you change your focus. This is another reason why binocular balancing is important in younger patients.